Determine the context of the current position.
By default, parse until we find a start-tag as the first thing on a line.
If UNTIL is `empty', return even if the context is empty (i.e.
we just skipped over some element and got to a beginning of line).
The context is a list of tag-info structures. The last one is the tag
immediately enclosing the current position.
Point is assumed to be outside of any tag. If we discover that it's
not the case, the first tag returned is the one inside which we are.